1. Field of the Invention
This invention relates to an anti-skid control apparatus for a vehicle baking system which can prevent the locking of the wheels.
2. Description of the Prior Art
The same assignee as this assignee previously proposed (U.S. Ser. No. 760,356 now U.S. Pat. No. 4,637,664) an anti-skid control apparatus for a vehicle braking system including a pair of front wheels; a first fluid pressure control valve device for controlling the brake fluid pressure of the wheel cylinder of one of the front wheels arranged between a first fluid pressure generating chamber of a tandem master cylinder and the wheel cylinder of the one front wheel; a second fluid pressure control valve device for controlling the brake fluid pressure of the wheel cylinder of another of the front wheels arranged between a second fluid pressure generating chamber of the tandem master cylinder and the wheel cylinder of the other front wheel; a control unit for measuring or judging the skid conditions of the front and rear wheels and for generating instructions for controlling the first and second fluid pressure control valve devices; and a valve apparatus for generating a fluid pressure in accordance with the lower one of the brake fluid pressures of the front wheels controlled by the first and second fluid pressure control valve devices, arranged between the wheel cylinders of the front wheels and those of the rear wheels.
For a counter-measure to the failure of the one of the two conduit systems in the above anti-skid control apparatus, a piston for opening and closing a pair of valve parts is slidably fitted into a sleeve. The fluid pressures of the fluid pressure generating chambers of the tandem master cylinder are applied to the end surfaces of the sleeve. When both of the two conduit systems are in order, the sleeve is located at the balanced neutral position. When one of the two conduit systems fails, the sleeve is moved in one direction, and so one of the valve parts arranged at either sides of the piston is forcibly maintained at the opened state. The piston is initially moved together with the sleeve, and then stopped by stopper means. The sleeve is further moved to maintain the one valve part at the opened state. Therefore, much fluid from the master cylinder is required for moving the sleeve and piston. Accordingly, the stroke of the brake pedal becomes large. Moreover, the above-described anti-skid control apparatus is complicated in construction.
Further, the counter-measure was proposed where the failure pistons are arranged at either side of the piston such that when one of the two conduit systems fails, one of the failure pistons is moved in one direction to maintain the one valve part forcibly at the opened state. However, in this counter-measure a large quantity of fluid is required for moving the one failure piston and the piston for opening and closing the valve parts.